Digital Camera Buyer's Guide: DSLR - Comments

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  • #2
DrClaude
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Nice Insight Andy.

One thing I wonder about is why there is still an "R" in DSLR, now that we don't use film. Is there any advantage to not having the sensor exposed all the time? I can't think of many disadvantages, like the mirror vibrations you discuss yourself.
 
  • #3
olivermsun
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Nice Insight Andy.

One thing I wonder about is why there is still an "R" in DSLR, now that we don't use film. Is there any advantage to not having the sensor exposed all the time? I can't think of many disadvantages, like the mirror vibrations you discuss yourself.
A few reasons (by no means exhaustive) why the reflex mirror/optical viewfinder still survives today:
  1. Optical viewfinder still has fastest response and potentially best color/resolution
  2. Off-sensor phase detect AF (receive their light through a semi-silvered reflex mirror and sub mirror) still best for moving/unpredictable subjects
  3. Similarly, off-sensor exposure meters (including TTL flash meters) can be useful
  4. Optical viewfinder doesn't eat batteries
Also, reasons not to have the sensor exposed all the time include heat/image noise, blooming, etc.

Obviously, EVFs have many unique advantages, and most of their disadvantages relative to a reflex mirror/optical viewfinder are diminishing as EVFs continue to improve.
 
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  • #5
Andy Resnick
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Nice Insight Andy.

One thing I wonder about is why there is still an "R" in DSLR, now that we don't use film. Is there any advantage to not having the sensor exposed all the time? I can't think of many disadvantages, like the mirror vibrations you discuss yourself.
Good question. If I understand you, the sensor can't be exposed all the time, because then there's no way to set an exposure time. Mirrorless cameras have an electronic mechanism to 'wipe' the sensor prior to an exposure, and the main advantage to mirrorless systems is as you say- there's no mirror that moves, not only reducing vibrations but also decreasing the distance between lens mount and sensor, allowing for smaller lenses. Mirrorless cameras can easily use lenses designed for rangefinder cameras.

On the other hand, mirrorless cameras have a decreased optical throughout, because some of the light is permanently re-directed to the (digital) viewfinder. I don't know exact numbers, but AFAIK, this represents about a half-stop of light lost.
 
  • #6
olivermsun
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On the other hand, mirrorless cameras have a decreased optical throughout, because some of the light is permanently re-directed to the (digital) viewfinder. I don't know exact numbers, but AFAIK, this represents about a half-stop of light lost.
Wait, doesn't the EVF usually get fed by the image sensor itself?
 
  • #8
olivermsun
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I expect different manufacturers have different approaches. Often, there is a pellicle beamsplitter that directs some light to the autofocus sensor:

https://qph.ec.quoracdn.net/main-qimg-4af617c02795fb9260f69dbaded8ddf9-c?convert_to_webp=true
That depicts the so-called SLT (Translucent instead of Reflex) configuration, used by Sony and previously by Canon to remove the need for flipping the mirror at a small cost to the light transmitted to the sensor. In other respects it is essentially the same as a traditional AF SLR and provides an optical viewfinder.
 
  • #9
Andy Resnick
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That depicts the so-called SLT (Translucent instead of Reflex) configuration, used by Sony and previously by Canon to remove the need for flipping the mirror at a small cost to the light transmitted to the sensor. In other respects it is essentially the same as a traditional AF SLR and provides an optical viewfinder.
Exactly.
 
  • #10
olivermsun
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Exactly.
Hmm, I don't understand why you chose this example then. The SLT is actually an SLR with a Pellicle mirror and an optical viewfinder, so it doesn't explain why mirrorless cameras should lose a fraction of a stop to the EVF.
 
  • #11
Andy Resnick
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Hmm, I don't understand why you chose this example then. The SLT is actually an SLR with a Pellicle mirror and an optical viewfinder, so it doesn't explain why mirrorless cameras should lose a fraction of a stop to the EVF.
I think you missed my point- my point is that the throughput is lower on a mirrorless camera as compared to either a reflex or rangefinder camera, it's less relevant where the re-directed light goes.
 
  • #12
olivermsun
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I think you missed my point- my point is that the throughput is lower on a mirrorless camera as compared to either a reflex or rangefinder camera, it's less relevant where the re-directed light goes.
I think I got your point, but I am disagreeing that the "throughput" of a mirrorless camera has to be any different from that of a reflex or rangefinder camera.

I replied about your SLT example because, while it's true that SLTs have lower throughput than a traditional SLR, they are not mirrorless cameras, so I don't think they are a relevant example.

Finally, I do think it's relevant to explain where the re-directed light goes. If, as on a modern mirrorless camera, the light isn't re-directed anywhere but it goes straight to the image sensor, just as it does in a digital rangefinder or an SLR in "live view" mode with the mirror flipped up, then why would the mirrorless camera have any lower throughput than the others?
 
  • #13
Andy Resnick
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I think I got your point, but I am disagreeing that the "throughput" of a mirrorless camera has to be any different from that of a reflex or rangefinder camera.
Maybe it would be helpful to identify the specific camera you are thinking about?
 
  • #14
olivermsun
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The Sony A6xxx and A7 series are current mirrorless cameras with very similar sensors to several (e.g., Nikon) DSLRs, and they show low-light performance very comparable to same-format DSLRs. Olympus m4/3 cameras also perform just fine, with allowance for the smaller sensor size.

However, empirical results aside, it's probably equally or more important to understand what are the fundamental constraints of various configurations and what creates those constraints.
 
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  • #15
Andy Resnick
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The Sony A6xxx and A7 series are current mirrorless cameras with very similar sensors to several (e.g., Nikon) DSLRs, and they show low-light performance very comparable to same-format DSLRs. Olympus m4/3 cameras also perform just fine, with allowance for the smaller sensor size.
Ah- this is helpful. As I said, different manufacturers implement technologies in their own way. The Sony A7 series embeds the AF sensor into the main image sensor, so there's no pickoff and no loss of light. Similarly, use of an electronic front curtain shutter allows for elimination of one of the mechanical shutters (I think a back curtain shutter is still required).

https://www.mhohner.de/newsitem2/efcs
 

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